To meet all of the emergency and service requirements of modern heavy-duty highway tractor-trailer combinations, while meeting all of the pertinent governmental regulations, including FMVSS 121, the trucking industry has resorted to a number of trailer semi-brake valve arrangements which have led to complexity, high costs, installation and maintenance difficulties and related problems. FMVSS 121 has recently been revised to require a minimum supply line pressure of 70 psig in service braking systems. A minimum governor cut-in pressure of 85 psig is also required by FMVSS 121.
The 70 psig minimum supply line requirement is typically met by adjusting a valve spring associated with a pressure protection valve that governs the pressure at which air is supplied to the spring brakes and to service brake air reservoirs. Because 100% compliance with the 70 psig supply line minimum is required, the pressure protection valve must be adjusted to maintain pressure in excess of 70 psig to ensure that the 70 psig minimum requirement is met in spite of worst-case stack-up of tolerances and small system leaks. In some cases, the pressure protection valve must be set to maintain a supply line pressure in excess of the 85 psig minimum governor cut-in pressure to ensure compliance with the 70 psig minimum supply pressure requirement.
If the pressure protection valve setting is higher than the minimum governor cut-in pressure, it is possible that the system pressure in a charging vehicle could be smaller than the setting of the pressure protection valve. In this case, it would be impossible to release the parking brakes or to fill the service brake reservoir of the vehicle to be charged. An improved full-function valve that overcomes this problem is desirable.